1. Field of the Invention
The invention relates to an electrophotographic printing system and, more particularly, to an electrostatically operative toner transfer system for an electrophotographic printing system.
2. Description of Related Art
Printers provide a means for outputting a permanent record in human readable form. Most printers may be characterized as utilizing either an impact printing technique or a non-impact printing technique. In impact printing techniques, an image is formed by striking an inked ribbon placed near the surface of a sheet of paper. Impact printing techniques may be further categorized as utilizing either formed-character printing or dot-matrix printing. In formed-character printing, the element which strikes the ribbon to produce the image consists of a raised mirror image of the desired character. In dot-matrix printing, the character is formed as a series of closely spaced dots which are produced by striking a provided wire or wires against the ribbon. By selectively striking the provided wires, any character representable by a matrix of dots can be produced.
Non-impact printing is often preferred over impact printing in view of its tendency to provide higher printing speeds as well as its better suitability for printing graphics and half-tone images. Non-impact printing is also relatively noise free. One form of non-impact printing is generally classified as ink jet printing. In this technique, ink is forced, most commonly under pressure, through a tiny nozzle to form a droplet. The droplet is electrostatically charged and is attracted to an oppositely charged platen located behind the sheet of paper. Using electrically controlled deflection plates similar to those in a CRT, the trajectory of the droplet can be controlled to hit a desired spot on the paper. Unused drops are deflected away from the paper into a reservoir for recycling the ink. Due to the small size of the droplet and the precise trajectory control, ink jet printing quality can approach that of formed-character impact printing.
Another non-impact printing technique is generally referred to as electrophotographic printing. In this technique, a controller, for example, a microprocessor associated with a computer system, turns a small laser or other light source on and off at a very rapid rate which readily exceeds millions of times per second. Reflection means, for example, a multifaceted polygonal mirror, reflects the light stream off a facet thereof and sweeps the light stream across a negatively charged print drum. The reflected light discharges portions of the surface of the negatively charged print drum, thereby producing a latent image on the drum. As the print drum rotates, it is dusted with negatively charged toner in the form of small particles. The toner particles stick only to the discharged areas. When negatively charged paper contacts the drum, the toner particles are attracted to the discharged image areas, thereby forming the desired image. The image is then fused to the paper by a combination of heat and pressure. As the finished page is produced, the drum is cleared of its electrical charge, cleaned and recharged for a next cycle.
Various techniques have been utilized to supply toner to a photosensitive surface, for example, the surface of a print drum, where a latent image has been formed. In cascade type developing devices, a developer material which is comprised of carrier material and toner particles is cascaded onto the photosensitive surface from a position above the surface, thereby developing a latent image previously formed on the photoreceptor surface into a visible toner powder image. The use of cascade type developing devices is often undesirable due to the size, slow process speed and somewhat reduced print quality of such devices, particularly when the reproduction of half-tones and solid areas is desired.
Many other techniques utilize magnetic attraction to supply toner to the photosensitive surface of a print drum. For example, U.S. Pat. No. 3,985,436 to Tanaka et al. discloses an electrophotographic copying system in which stationary bar magnets attract developing material consisting of magnetizable carrier material such as iron particles and electroscopic toner particles onto the surface of a cylinder to form magnetic brush bristles. As the cylinder rotates, the brush bristles rub lightly against the latent image formed on the surface of the rotating drum to develop the latent image into a visible toner powder image.
While electrophotographic printing techniques which utilize magnetic developers produce high quality images at relatively fast processing speeds, there remain several drawbacks to such systems. In particular, the brushing action on a print drum produced by a developer roller carrying a developer which includes both toner and carrier tends to wear itself out as well as wear out the print drum. Numerous solutions to this problem have been proposed. For example, U.S. Pat. No. 4,538,896 to Tajima et al. describes a electrophotographic copying system where a hopper supplies a one component magnetic toner to a magnetic roller. In turn, the magnetic roller transfers the toner to a photosensitive drum.
Other solutions avoid the use of magnetic developers entirely. For example, U.S. Pat. No. 4,100,884 to Mochizuki et al. discloses an apparatus for developing an electrostatic image on a photosensitive member in which a one component nonmagnetic toner is supplied to a developer roller. As the surface of the developer roller is formed of an elastic rubber having good adherence and retention qualities for fine powders, the toner is readily attached to the surface of the developer roller as a toner layer. In the event that the deposited toner is not level, a levelling member is included to provide a uniform thickness for the toner layer. The apparatus disclosed by Mochizuki et al. further includes a triboelectric charging member which charges the retained toner to a given polarity. As the electrostatic attraction is greater than the toner retaining capability of the developer roller, the charged toner will then be deposited on the image bearing member.
It is an object of this invention to provide an electrophotographic printing system which includes an electrostatically operative toner transfer system.
It is another object of this invention to provide an electrostatically operative toner transfer system which utilizes relative differences in electrical charges to promote the transfer of toner through the toner transfer system.
It is yet another object of this invention to provide an electrophotographic printing system in which electrostatic attraction is utilized to orientate the toner, thus electrostatically simulating a magnetic brush-like configuration.